Recently, there has been a need for improvement of fuel economy of motor vehicles. Particularly, there has been a need for a system that can reduce friction loss by early warming up cooling water, an engine oil, ATF (Automatic Transmission Fluid) or like in order to prevent deterioration of fuel economy when the engine is cold, such as when the engine is started. Further, there has been a need for a system that heats a catalyst in order to activate a catalyst for cleaning an exhaust gas at an early stage.
An example of such a system is a heat exchanger. The heat exchanger is a device including a component (a heat exchange component) that performs heat exchange by allowing a first fluid to internally flow and a second fluid to externally flow. In such a heat exchanger, heat can be effectively utilized by heat exchange from a higher temperature fluid (for example, the exhaust gas) to a lower temperature fluid (for example, the cooling water).
Patent Document 1 discloses a heat exchange member capable of improving fuel economy of a motor vehicle when recovering waste heat from an exhaust gas and using the heat for warming up an engine in the field of the motor vehicle. However, the heat exchange member disclosed in Patent Document 1 has a structure in which waste heat is always recovered from the first fluid (for example, the exhaust gas) to the second fluid (for example, the cooling water), so that the waste heat may be recovered even if it is not necessary to recover the waste heat. Therefore, it has been necessary to increase a capacity of a radiator for releasing the recovered waste heat when it is not necessary to recover the waste heat. Further, there has been a problem that an increased amount of heat for heat exchange from the first fluid to the second fluid may boil the second fluid (for example, the cooling water).
Patent Document 2 describes a heat exchanger for recovering heat of an exhaust gas of an engine. The heat exchanger suppresses boiling and evaporation of the cooling water for the engine when the heat of the exhaust gas from the engine is recovered into the cooling water. The heat exchanger described in Patent Document 2 includes an exhaust gas passage and a first medium passage adjacent to each other through a second medium passage interposed therebetween, and is configured to fill the inside of the second medium passage with the liquid phase second medium when promoting the heat exchange between the exhaust gas and the first medium. Thus, according to the heat exchanger described in Patent Document 2, the heat exchange can be gently promoted while suppressing the boiling and vaporization of the first medium by heat exchange using the convection of the liquid phase second medium, as compared with a case where direct heat exchange is performed not through the second medium. Further, the heat exchanger is configured to fill the inside of the second medium passage with a gas when suppressing the heat exchange between the exhaust gas and the first medium. Therefore, according to the heat exchanger, the boiling and vaporization of the first medium can be further suppressed as compared with the heat exchange through the liquid phase second medium as described above.